The present invention relates generally to bearings.
Foil bearings, such as disclosed in U.S. patent application Ser. Nos. 08/827,203 and 08/827,202 (now U.S. Pat. Nos. 5,902,049 and 5,833,369 respectively), assigned to the assignee of the present invention, and U.S. Pat. Nos. 4,262,975; 4,277,113; 4,300,806; 4,296,976; 4,277,112; 4,277,111; 5,833,369; and 5,902,049 of Hooshang Heshmat (either as sole or as joint inventor), an inventor of the present invention, which applications and patents are incorporated herein by reference, include a sheet positioned to face a shaft portion for relative movement there between and means in the form of a corrugated shape having a plurality of ridges or other suitable form for resiliently supporting the sheet thereby defining a compliant hydrodynamic fluid film bearing. The bearing may be a journal bearing in which case the sheet is in surrounding relation to a shaft for relative rotational movement there between or a thrust bearing in which case the sheet bears a rotating shaft runner. The bearing axis may alternatively be slanted to the radial and axial directions and therefore have the attributes of both a journal and thrust bearing. Stiffness and damping are provided in a foil bearing by the smooth top foil or sheet and structural support elements which are suitably designed to provide a compliant spring support of the desired stiffness (or stiffness which is variable with load) and damping and by the hydrodynamic effects of a gas film between the shaft and the smoothtop foil.
Magnetic bearings, such as disclosed in U.S. patent application Ser. No. 09/046,334, which is assigned to the assignee of the present invention, and in U.S. Pat. Nos. 5,084,643; 5,133,527; 5,202,824; and 5,666,014, which applications and patents are incorporated herein by reference, include magnet means on a housing which magnetically interact with a shaft portion for adjusting the position thereof during rotation thereof. A magnetic bearing may be provided as either a journal or a thrust bearing.
Magnetic bearings may be classified as using either repulsive or attractive forces. Repulsive force systems often use permanent magnets while attractive force systems usually use electromagnets. Attraction electromagnets are usually used for magnetic suspension systems (bearings) since stiffness nearly comparable to rolling element bearings can be achieved and since active control permits variation of parameters as dictated by rotor system dynamics. An actively controlled magnetic bearing generally comprises a stator which is wound with coils to create the magnetic field and ferromagnetic laminations mounted on the rotor to interact with the stator magnetic field.
Position sensors have conventionally provided feedback for control of magnetic bearings. Bias currents are conventionally applied to the electromagnets to support static loads and set up an operating flux field for linearized control. Since the flux field is equivalent to a negative spring, the bearing is inherently unstable. For reliable rotor control, both rotor position and its rate of change need to be corrected. In other words, the active magnetic bearing needs damping or velocity control, which has been conventionally achieved by feedback of the derivative of the displacement signal. In addition to dynamic stiffness and damping, basic rotor position error feedback is required to statically center the rotor. A typical magnetic bearing control is thus a gain and phase compensation network which provides a summation of (1) the time-varying position signal for dynamic stiffness control (which may be called xe2x80x9cProportionalxe2x80x9d), (2) the integral of the position signal error for static stiffness control (which may be called xe2x80x9cIntegralxe2x80x9d), and (3) the derivative of the time-varying position signal for damping (which may be called xe2x80x9cDerivativexe2x80x9d). High static stiffness is provided to keep the rotor centered in the bearing. With independent control of each of these elements, such a conventional controller, which may resultingly be called a xe2x80x9cPID controllerxe2x80x9d, allows the magnetic bearing characteristics to be varied as a function of machine operation. Lead-lag or notch filter circuits are added to the PID circuit to allow gain and phase compensation at resonant and other frequencies not covered by the PID circuit. Common rotordynamic controls include varying the bearing stiffness to alter lateral vibration modes, inserting damping to reduce dynamic motion, and generating rotating bearing forces to oppose or cancel rotor unbalance and harmonic forces. Other types of controllers such as, for example, fuzzy-logic controllers have also been used for magnetic bearing control.
The mechanical simplicity of foil bearings makes them suitable for high-speed machines such as those with cryogenic turbo-rotors with both expander and compressor wheels running at high speeds, i.e., on the order of tens of thousands of rpm. However, a significant effort is required to design a set of foil bearings for any new application. Furthermore, foil bearings do not lift off at low speed, thus requiring a coating on the foil for protection thereof at low speeds during start-ups and shut-downs. To make a long-lasting coating, uniform foil surface compliancy must be provided by design. Moreover, it is not easy to design an adequate amount of damping in the foil bearing crucial for rotor stability at high speeds.
Active magnetic bearings have many advantages. Among these are non-contact shaft support at start-up and low shaft speeds and electrically adjustable dynamic characteristics. However, difficulties can arise in such bearings with control of rotor and/or structural resonances due to controller limitations and/or sensor-actuator non-collocation (sensors not in same axial locations as the actuator). High frequency modes and sensor noise can also result in power amplifier saturation. Furthermore, it is difficult to provide reliable and long-lasting back-up bearings for active magnetic bearings. Conventional rolling-element type back-up bearings tend to have skidding wear and last for only a few rotor drops due to electric failures. Moreover, violent backward whirl may occur to render a rotor-bearing failure a disaster. However, some progress is being made to provide improved backup bearings.
Since the foil bearing is considered to be advantageous for high speed operation and the magnetic bearing for low-speed operation, it is considered advantageous to combine them into a hybrid bearing having the advantages of each, wherein the load is shared between them. See H. Heshmat (one of the inventors of this application) et al, xe2x80x9cHybrid Foil-Magnetic Bearings,xe2x80x9d the 7th Nordic Symposium on Tribology (NORTRIB""96), Bergin, Norway, Jun. 16-19, 1996. See also Canadian patent 2,151,687 to Heshmat (one of the inventors of this application) et al.
The hybrid journal bearing is considered to provide, for some applications such as aircraft gas turbines, the following benefits. Since the specific capacity of a foil bearing is typically about 500 lbs. per lb. of bearing weight and since that of the active magnetic bearing is typically about 40 lbs. per lb. of bearing weight, the hybrid bearing should be much smaller, lighter in weight, and consume less power than a pure active magnetic bearing, for the same load capacity. The rotor may coast down safely on the foil bearing part in case of electric power loss to the magnetic bearing part. The foil bearing coating wear problem in such a hybrid bearing is no longer a problem because the magnetic bearing part can take the load at low speeds. Rotor system stability can be enhanced by tuning of the magnetic bearing control algorithm. For example, the magnetic bearing may add damping to reduce the potential for sub-synchronous instabilities. The tuning of the magnetic bearing part in high frequency range may be simplified because the rotor is supported by the foil bearing part, which provides additional damping. The ability to independently vary bearing characteristics, provided by the active magnetic bearing, offers versatile rotor control. For an active magnetic bearing, the rotor speed has no direct effect on the load capacity.
The benefits of a hybrid foil-magnetic thrust bearing are considered to be similar to those of a hybrid foil-magnetic journal bearing. Thus, the hybrid thrust bearing for high-speed and high-temperature applications would carry more load per pound of bearing weight than a conventional thrust active magnetic bearing. The hybrid thrust bearing would have superior dynamic characteristics because the foil part, inherently a high speed bearing part, and the magnetic part, which with solid cores performs well at low frequencies, would tend to complement each other. The foil part, since it would not take up any load at start-ups and shut-downs, would not need a coating or its coating, if provided, should last a long time. Furthermore, at high speeds, the foil part would be able to take over and prevent bearing catastrophe in case of electric or control failures.
A foil bearing may of course be provided solely as a back-up bearing for use only in allowing the rotor to coast safely to a stop in the event that the magnetic bearing fails. Of course the foil bearing of the present invention would, in addition to sharing load with the magnetic bearing, also be able to act as a back-up in the event of magnetic bearing failure.
Other benefits of a hybrid foil-magnetic journal or thrust bearing over either a foil or magnetic bearing individually include (but are not limited to) (1) improvement of the life of the foil bearing surface through the magnetic bearing""s capability to provide load support while the rotor is not rotating or while the foil bearing does not provide adequate load capacity, (2) increased load capacity over either bearing alone, (3) ability to implement active control algorithms via the magnetic bearing control logic to alter the bearing system performance for goals such as vibration cancellation, critical speed modifications, gain scheduling to provide different characteristics for different operations, etc., and (4) reduced sensitivity to mechanical misalignment between the two bearings, including that due to thermal growth.
Such a hybrid bearing is provided to combine the high speed load capacity and shock tolerance of the foil bearing with the zero speed load capacity and active control capabilities of an active magnetic bearing as well as to effectively triple or increase greatly the load capacity of the magnetic bearing and to eliminate concerns about foil bearing/shaft rubbing at low speeds. It is considered desirable, in order to obtain the above benefits, to control the amount of load shared by each bearing in such a hybrid journal or thrust bearing. However, in a hybrid journal bearing, there is an eccentricity of the foil bearing part, as seen in FIGS. 3 and 4, which is discussed hereinafter, which would seem to be incompatible with the lack of such eccentricity in a conventional magnetic bearing. Thus, these eccentricity differences, wherein the natural rotational center of a rotor within a magnetic bearing would be different from its natural rotational center within a foil bearing, would seem to rule out a hybrid use of both a foil bearing part and a magnetic bearing part.
At the operating speed, the load on a thrust bearing varies from a minimum or normal thrust load to a maximum due to, for example, a compressor surge. The hybrid thrust bearing should be able to take any load amount within its capacity without xe2x80x9cthinkingxe2x80x9d (performing off-line logic calculations and making decisions in a supervising controller to re-set parameters) since there may not be enough time to do the xe2x80x9cthinkingxe2x80x9d.
The above Heshmat et al paper discloses an indirect off-line load-sharing control method wherein a control reference center for the magnetic bearing element is changed to correspond to a respective rotor center for the foil bearing element at each of various rotor speeds.
While such an indirect load-sharing method has been found to work well, it is considered desirable to provide direct load-sharing ability for more quickly responsive and otherwise improved bearing performance including the ability to reduce foil bearing load to a minimum for reduced foil bearing wear during start-up and low-speed operation, regardless of any misalignment between the bearings, and to maintain stable bearing operation.
It is accordingly an object of the present invention to provide a hybrid foil-magnetic bearing wherein the load-sharing is directly controllable for more quickly responsive and otherwise improved bearing performance as described above.
In order to achieve such directly controllable load-sharing, in accordance with the present invention, there is provided a hybrid foil-magnetic bearing wherein actual load on at least one of the foil and magnetic bearings is sensed and inputted to a controller for sharing of the load between the foil and magnetic bearing parts.
The above and other objects, features, and advantages of the present invention will be apparent to one of ordinary skill in the art to which the present invention pertains in the following detailed description of the preferred embodiments of the present invention when read in conjunction with the accompanying drawings wherein the same reference characters denote the same or similar parts throughout the several views.